Question

An electrically isolated hollow (initially uncharged), conducting sphere has a small positively charged ball suspended by an insulating rod from its inside surface, (see diagram).

This causes the inner surface of the sphere to become negatively charged. When the ball is centred in the sphere the electric field outside the conducting sphere is

• A. zero
• B. the same as if the sphere wasn't there
• C. twice what it would be if the sphere wasn't there
• D. equal in magnitude but opposite in direction to what it would be if the sphere wasn't there

Answer 1

Answer: (B)

Let the charge on the ball at centre is $q$, then $-q$ charge will be induced on the inner surface and $+q$ on the outer surface. Field at any point outside the sphere due to inner charges $q$ and $-q$ will jointly be zero.

Outer charge $q$ will produce the electric field at outside points as it would be produced by a charge at the centre. Hence, presence of sphere does not affect the electric field outside the sphere.